Health regulators in the U.S. and Europe have added warnings to the Multiple Sclerosis (MS) drug Gilenya to indicate its associated risks to patients with cardiovascular and cerebrovascular disease.

The move by the Food and Drug Administration and European Medicines Agency was prompted by reports of an ongoing trial of Gilenya in which 15 patients died. The warning includes risks specific to patients with cardiovascular and cerebrovascular disease or to patients who are already taking a drug that lowers their heart rate.

Gilenya is a ground-breaking drug in the treatment of MS in that it’s the first drug available in pill form. It already has some fledgling competition on the market but remains the top-selling pill MS drug. At least 36,000 people are now prescribed Gilenya worldwide.

Novartis, the makers of Gilenya, have responded to the news of the label changes for its drug positively and believe the FDA and EMA’s ongoing reviews of the benefits-risks comparison of it will work in the company’s favor.

The FDA suggests patients taking their first dose of Gilenya be monitored during the first six hours of the treatment to note any signs that complications are going to happening or if they are occurring. For patients with a slower heart rate after those six hours, they should be closely monitored for two more before OK’ing a prescription for the drug.